A backbias voltage generator which is a kind of the voltage generator used in a conventional semiconductor memory device is disclosed in U.S. Pat. No. 4,775,959.
As shown in FIG. 2, this backbias voltage (also called Vbb) generator includes a first Vbb generator Vbb-G1 and a second Vbb generator Vbb-G2.
The first Vbb generator Vbb-G1 includes a first oscillating section OSC1 and a rectifying section REC, while the second Vbb generator Vbb-G2 includes a second oscillating section OSC2 and a rectifying section REC.
The first and second Vbb generators Vbb-G1 and Vbb-G2 are composed of voltage generating circuits, one of which is shown in FIG. 1.
As shown in this drawing, the first inputs of the three NAND gates are connected in series, and the output of the last NAND gate is connected to the input of the first NAND gate, so that the oscillation circulating signals are circulated, thereby forming an oscillator. The output of this oscillator is supplied through a buffer circuit 12 to a first electrode of a pumping capacitor C1. A second electrode of this capacitor is connected to a rectifier which consists of transistors 16 and 17 (each of which serve as a rectifying element). Further, the second inputs of the three NAND gates NAND1, NAND2 and NAND3 are commonly connected together to receive oscillator enable signalEN.
The capacitor of the second Vbb generator Vbb-G2 has a large capacitance, while the capacitor of the first Vbb generator Vbb-G1 has a relatively small capacitance. The voltage generating operation of this circuit is carried out in such a manner that, if the internal power source Vcc is supplied, the oscillator enable signals are supplied to the second inputs of the NAND gates to activate the oscillator OSC so that oscillations start, and that an oscillating output is generated.
If the oscillation output signals have a high level, the signals make first electrode 14 of the pumping capacitor have a high level after passing through a buffer circuit 12 which drives the pumping capacitor. A second electrode 15 of the pumping capacitor is also made high by the coupling effect, thereby raising the voltage.
If the potential of the second electrode 15 of the pumping capacitor is higher than a positive terminal (e.g., ground potential), then a first rectifying device 16 is made conductive, so that its drain voltage becomes the same as that of the positive terminal.
Then, if the oscillation output signals are shifted to a low level, the signals make the first electrode 14 of the pumping capacitor have a low level after passing through the buffer circuit 12 which drives the pumping capacitor. At the same time, the second electrode 15 of the pumping capacitor becomes low by the coupling effect.
The potential of the second electrode 15 of the pumping capacitor become lower than that of the positive terminal, with the result that the first rectifying device 16 is made nonconductive. If the potential of the second electrode 15 of the pumping capacitor becomes lower than the potential of a negative terminal (e.g., backbias potential), then the second rectifying device 17 is made conductive, so that its potential becomes the same as that of the negative terminal.
Then, the oscillation output signals are shifted to a high level again to repeat the above operations, with the result that the electrons of the positive terminal are pumped by being transferred to the negative terminal, thereby generating a voltage.
The amount of negative voltage generating power by the first Vbb generator Vbb-G1 is very weak, but it is designed that the generated voltage is sufficient to compensate the leakage of the transistors during a standby mode in which the chip is not operated.
Meanwhile, the amount of the negative voltage generating power by the second Vbb generator Vbb-G2 is much larger, so that it is able to compensate the leakage of the transistors during the operation of the semiconductor device. If the voltage generation is made to be large, the capacity of the buffer circuit which drives the pumping capacitor has to be made large, as well as increasing the capacities of the pumping capacitor and the rectifying device.
In the circuit of FIG. 2, if the negative voltage is continuously supplied until the voltage level of the Vbb drops to below a certain predetermined value, then a backbias voltage detecting section (VLD section) emits a backbias voltage detecting signal, so that the voltage generator enable signal shifts from high to low, thereby maintaining the voltage at a constant level.
In this conventional technique, when the chip is in the standby state, most transistors are turned off, and only equalizers and some transistors operate, so that the leakage rate is relatively small. Under this condition, only the first Vbb generator Vbb-G1 which has a small driving capacity is driven, so that the power consumption can be reduced. Meanwhile, when the chip is active, or when the Vbb voltage level is below the predetermined level (-3 Vth), the second Vbb generator Vbb-G2 which has a large driving capacity is driven, so that the increase of the Vbb voltage due to the large current leakage through the driving of a large number of transistors can be prevented.
The above described voltage generator for use in a conventional semiconductor memory device has a fixed oscillation period, and, therefore, it cannot accurately operate against the current leakage in a plurality of transistors in which the operation conditions are varied in the case of the active and standby conditions. That is, the oscillating frequency is decided by calculating the average of the current leakage. Further, the peak current for driving the large pumping capacitor becomes very high, thereby causing large voltage variations, as well as aggravating the reliability of the semiconductor memory device.